A variety of toxins injure cells in a manner which suggests that hey may reduce molecular oxygen as a necessary step in their cytotoxic actions. These include X-rays, phagocytes, some cancerocidal drugs, and molecular oxygen itself. In one prototype of toxicity fostered by the reduction of molecular oxygen, the various cytotoxic actions of paraquat, mutagenicity and malignant transformation were observed but whether either of these was due O2-, H2O2, or OH was not clear. To determine whether the mutagenic and transforming effects of paraquat are caused by its ability to promote the formation of O2- or H2O2 cultured cells will be enriched with superoxide dismutase, with catalase, and with glutathione peroxidase by transcription of transfected cDNAs. Cells thus enriched will serve as experimental tools in which to compare the mutagenic and transforming effects of paraquat with the effects of paraquat in control cells not enriched for any of these three enzymes. The converse experiment, to deplete cells of each of these enzymes, will also be performed. If cells enrich for catalase resist the mutagenic effect of paraquat whereas cells depleted of catalase are more sensitive to mutagenesis by paraquat, a role for H2O2 in the mutagenic action of paraquat is suggested. To determine whether O2- and H2O2 themselves act in the nucleus I shall compare the mutagenic effects of paraquat in cells in which the nucleus is enriched for each of these three enzymes (superoxide dismutase, catalase, and glutathione peroxidase) with the same cells in which the enzyme remains in the cytosol. The means for driving enzymes into the nucleus is fusion of each enzyme with the portion of the glucocorticoid receptor which binds glucocorticoids and translocates proteins of which it forms a part into the nucleus. Finally, I shall determine the means by which paraquat- resistant HeLa cells have increased their content of superoxide dismutases in less resistant cells and of catalase and glutathione peroxidase in more resistant cells. An understanding of the means by which this occurs may indicate how the cellular content of these enzymes is regulated in mammalian cells.